The present invention relates to stacked microelectronic assemblies, to methods of forming such assemblies, and stackable microelectronic packages useful in such stacked assemblies.
Microelectronic devices, such as semiconductor chips, piezoelectric elements and micromechanic systems, etc., are typically are thin and flat, with relatively large front and rear surfaces and small edge surfaces. The devices have contacts on their front or rear surfaces. Typically, microelectronic devices are provided as packaged devices having terminals suitable for connection to an external circuit. Packaged devices, such as chips, typically are also in the form of flat bodies. Ordinarily, the packaged chips are arranged in an array on a surface of a circuit board. The circuit board has electrical conductors, normally referred to as “traces” extending in horizontal directions parallel to the surface of the circuit board and also has contact pads or other electrically conductive elements connected to the traces. The packaged chips are mounted with their terminal-bearing faces confronting the surface of the circuit board and the terminals on each packaged chip are electrically connected to the contact pads of the circuit board.
The conventional packages for microelectronic devices, such as chips, typically have a surface area that is larger than the area of the chip, causing the wiring board of interconnection to have excessively large area relative to the chips. However, as the operational speed of the device increases, it is desirable to move the chips close together since long signal paths deteriorates signal consistency and propagation times, and causes more electromagnetic noise. Other characteristics of the chip package such as manufacturing cost, reliability, heat transfer, moisture resistance and testability are also significant.
These considerations for microelectronic device packages have been addressed by the design of high-density packages, such as ball grid arrays (BGA) and chip scale packages (CSP). Although these packages provide certain advantages, further enhancement would be desirable.
Multi-chip modules (MCM) that package multiple chips on a common wiring board or substrate are an alternative for high-density packaging. These modules aim to achieve higher packaging density, and can also produce better signal quality, and reasonable manufacturing cost. Many MCMs are two-dimensional structures with multiple chips connected to a planar interconnection substrate which contains traces to supply power and signal transmission. However, since multi-chip modules utilize a planar interconnection substrate as a base element, their effectiveness in packaging density is limited.
In order to create higher density packages, reduce area requirements and shorten signal transmission distances, stackable device packages with two, three or more vertically stacked chips have been proposed. Stackable packages are particularly suitable for the electronic systems such as high performance parallel computing and large cache memory devices which require high operating speeds and high memory capacity in a limited space.
Examples of stacked packages are shown, for example, in U.S. Pat. Nos. 4,956,694; 5,198,888; 5,861,666; 6,072,233; and 6,268,649. The stacked packages shown in certain embodiments of these patents are made by providing individual units, each including a single chip and a package element having unit terminals. Within each unit, the contacts of the chip are connected to the unit terminals. The units are stacked one atop the other. Unit terminals of each unit are connected to the corresponding unit terminals of other units. The connected unit terminals form vertical conductors of the stacked package, also referred to as buses.
Despite all the advancements in technology for stacked microelectronic packages, still further improvements would be desirable. For example, it would also be desirable to provide a compact stacked package using readily-available equipment and using components that can be fabricated readily and with low cost. Furthermore, it would also be desirable to provide a stacked package with good heat transfer from the chips within the stack to the external environment as, for example, to the circuit board or to a heat spreader overlying the top of the package. Further, it would be desirable to provide such a package.